Conventionally, in power distribution systems such as those used in aircraft, each load circuit incorporates both a relay for switching and a thermal circuit breaker to protect the circuit wiring such circuit breakers are covered by military specification Mil-C-5809/1 if they have a dual or safety fuse feature. Also referenced in these circuit breaker specifications are trip curves designated MS3320 made from tabulated results for percent current overload versus time. The relay and circuit breaker for many circuits are located in cockpit for flight crew operation requiring heavy gage wire to run from the generator to the cockpit and then to the load resulting in a substantial weight penalty. A solid state power controller of the type shown and described in copending application Ser. No. (Attorney Docket No. 18437) assigned to the assignee of the present invention replaces both the relay and the circuit breaker in the power distribution system, providing both functions in a single device. It can also be remotely mounted to decrease wire weight and computer controlled to reduce flight crew workload.
Thermal circuit breakers incorporate a bimetallic element to sense the resistive heating of wire and cable due to current flow therethrough. A key inherent characteristic of the bimetal is "thermal memory". The bimetal stores and dissipates the heat energy in the same manner as the circuit wire and cable, and reduces circuit breaker trip time when the wire temperature remains elevated from a previous overload condition. This protects the wire and cable when exposed to multiple overloads or preload conditions by maintaining the heat energy below the threshold for wire damage. The thermal circuit breakers also allow momentary high inrush currents such as motor loads, charging capacitive loads and lamp loads without nuisance tripping. These circuit characteristics are incorporated in the referenced solid state power controller which functions as a thermal analog to wire and cable by following the MS3320 trip time versus overload current relationship while also performing relay functions. The MS3320 time-current curve parallels standard wire damage curves, enabling optimization of wire size and ampere rating. Additionally, during a short circuit or high level overload, the controller limits the circuit current to a preset limit and maintains it, tripping only when the rated maximum safe allowable temperature of the controller is reached.
In the event that the power controller does not operate as intended there is a need to provide a back-up safety mechanism to ensure that the protected circuits are not damaged by an overload condition without interfering in the normal operation of the power controller.
The power controller described and claimed in the above referenced application comprises a substrate mounting hybrid components in a package which, for a single controller, is in the order of two inches in length, one and a third inches in width and a third of an inch in height.
It is known to use fuse devices as a back-up safety mechanism for various electrical apparatus; however, such conventional fuse devices are not suitable for use with power controllers of the type described above. Typical prior art fuses do not conform to the MS 3320 time-current curve and would blow out in response to various overload conditions which by design the controller allows. The purpose of the back-up safety device is to blow out only in the event that the controller malfunctions and not to interfere with the normal operation of the power controller. Other devices which may conform to the MS3320 time-current curve are either too large in size to fit within the controller package or are not compatible with the surface mount assembly techniques.